In the prior art, the axial scanning methods include a mechanical scanning method, a remote focusing control method, a depth-of-field extension method, etc. In the mechanical scanning method, mechanical scanners are used to perform laser axial scanning. i.e., to move the objective lens or the specimen. However, the speed of mechanical scanning process is limited by the objective lens' inertia. For example, the speed of mechanically scanning the objective lens of a microscope using a piezoelectric actuator is limited to 10s Hz due to the weight of the objective lens.
In the remote focusing control method via electrical tunable lens (ETL) or spatial light modulators (SLM), the focal length of ETL can be electrically tuned by applying different current, and the spatial light modulators can be encoded with a standard lens' phase pattern to control the focus. However, the pattern update rate of an SLM is on the scale of 100 Hz, resulting in low speed. In the depth-of-field extension method, a wavefront encoder element or system may be used, or there exists an increase of spherical aberration with external medium, wherein a lens with extended focal length may be used by controlling spherical aberration. However, this could only be implemented in an imaging system as the real focal position is not controlled precisely and time-consuming post processing steps may be needed when the scanning range is large, i.e., deconvolution, to obtain high quality images.
The axial scanning methods in the prior art are either too slow (limited by the inertia of the objective lens or specimen) or have limited speeds, e.g., an electrical tunable lens (ETL) can scan only up to 100s Hz.
There is therefore a need for a practical approach to address at least one of the abovementioned problems.